The disclosure of Japanese Patent Application No. 2002-14804 filed on Jan. 23, 2002 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an inverter used in an alternating current generator-motor that is provided with both a generator function and a motor function.
2. Description of Related Art
There is a known type of vehicle that does not perform idling of an engine in a stopped state of the vehicle. The vehicle starts the engine with an extremely quiet starter motor each time it pulls away. This type of the vehicle generally employs an alternating current generator-motor as the extremely quiet starter motor. The alternating current generator-motor functions as a generator or an alternator while the vehicle is running normally. When the alternating current generator-motor is made to function as a motor or a starter motor, an inverter is required for converting a direct current power supply of a battery to alternating current and carrying out supply to the alternating current generator-motor. Furthermore, when the alternating current generator-motor is made to function as the generator (alternator), a rectifier is required to convert generated alternating current power to direct current power and store it in the battery. Therefore, a rectifying function for electricity generation is generally added to the inverter for the alternating current generator-motor.
Japanese Patent Application Laid-Open No. 10-191691 discloses an example of a known inverter for an alternating current generator-motor that can be applied to a vehicle and the like. As disclosed in the publication, a power control device includes multiple groups of switching elements and diode elements that are respectively connected in parallel, and a thermistor that detects overheating of the diode elements. The power control device controls an inverter module for controlling drive of a generator-motor and each of the switching elements of the inverter module. Then it is determined whether the diode elements have overheated on the basis of an output of the thermistor. The power control device includes a switching control circuit that suppresses or stops electricity generation of the generator-motor if it is determined that overheating has occurred so as to control drive and electricity generation of the three phase alternating current type generator-motor.
According to the aforementioned type of the inverter, if the diode elements overheat while the alternating current generator-motor is operating as the generator, electricity generation is suppressed so as not to cause failure in the switching elements connected to the diode elements. Energy efficiency, however, may be reduced because electricity generation is suppressed, resulting in insufficient power generation.
It is an object of the invention to provide an inverter for an alternating current generator-motor that is able to carry out sufficient electricity generation while preventing overheating of a diode that is formed on a switching element, while the alternating current generator-motor is made to operate as a generator.
An inverter for an alternating current generator-motor connected between the alternating current generator-motor and a charging-discharging unit is provided with a plurality of switching elements, each of which is controlled to convert a direct current electricity supplied from the charging-discharging unit into a three phase alternating current electricity so as to be supplied to the alternating current generator-motor that is functioning as a motor, a plurality of body diodes connected to the plurality of switching elements correspondingly. Each of the plurality of body diodes is operative to convert the alternating current generated by the alternating current generator-motor that is functioning as a generator into the direct current so as to be supplied to the charging-discharging unit. The inverter further includes a plurality of rectifying diodes connected in parallel with the corresponding plurality of body diodes, and at least one current controller that suppresses a flow of an electric current to each of the body diodes when the alternating current generator-motor generates electricity.
In one preferred embodiment of the invention, a current flowing in a body diode may be reduced. When this occurs, because a current is able to flow in a rectifying diode, it is possible to carry out sufficient electricity generation while preventing overheating due to overcurrent in the body diode.
Preferably the current controller is operative to interrupt a flow of the current directed to each of the plurality of body diodes when the alternating current generator-motor generates electricity.
In the preferred embodiment of the invention, the body diode does not generate any heat whatsoever because the current does not flow in the body diode even during electricity generation.
It is preferable to provide the inverter with a current detector that detects a current flowing to the charging-discharging unit when the alternating current generator-motor generates electricity. An amount of generated electricity is controlled in accordance with a detected value of the current detector.
In general, control of a charging current for a charging-discharging unit is achieved by controlling an exciting current in response to a voltage of the charging-discharging unit. If a state of charge of the charging-discharging unit is low, however, the voltage will not rise even if the charging current flows, and the current will continue to increase until the voltage rises. In the above case, however, if the amount of electricity generation is controlled in response to a detection value of a current detector, overcurrent can be prevented even when the voltage of the charging and discharging unit is low.
The current detector may be formed as a multi-functioning switching elements on a current path between the charging-discharging unit and the rectifying diode, formed to calculate a current value on the basis of a potential difference generated in a predetermined section on the current path between the charging-discharging unit and the rectifying diode, or formed as a transistor connected in parallel with the rectifying diode.